The present invention relates generally to a data acquisition system, and more particularly to an improved data acquisition system for use in simultaneously converting a plurality of related analog information signals to corresponding digital signals.
Various signal processing techniques involve the simultaneous detection of a plurality of analog information signals for the purpose of acquiring data represented by the signals. For example, certain commercially-available medical imaging systems, such as C.T. scan systems, are used to image internal features of an object under view by exposing the object to a preselected amount and type of radiation. Detectors sense radiation from the object and generate analog signals representative of internal features of the object.
In the example of a C.T. scan system, the X-ray source and detectors are rotated in a plane, relative to an object, through a series or sets of views or readings so that the system can acquire data representative of two-dimensional spatial information of a cross-sectional "slice" of the object lying in the plane of rotation of the source and detectors. These C.T. scan systems each utilize a source of X-ray photons. The source may provide periodic X-ray pulses, or alternatively, continuous-wave (CW) X-rays. The detectors, usually in the form of gas or solid state detectors, are disposed relative to the source so as to define a corresponding plurality of X-ray paths for each set of readings. When the object is exposed to the X-ray source, the number of photons absorbed along the various paths through the object, during each sampling period defining each view or reading, is a function of the absorption characteristics of the portions of the object along each path. The detectors generate a corresponding plurality of analog signals representative of X-ray flux detected during each sampling period.
While the signals generated by the detectors through the series of readings provide the required data to generate the 2-dimensional image, acquiring and processing the data can pose various design problems. For example, a large number of detectors must be used for each set of readings made during each sampling period in order to create a detailed image with sufficient resolution (a typical C.T. scan system contains of the order of 500 detectors, although the number can clearly vary). In order to improve the resolution of the image created, the number of detectors used and/or the number sets of readings must be increased. However, the greater the number of detectors and/or sets of readings utilized, the greater the amount of data acquired and the greater amount of signal information that must be processed. Accordingly, the analog signals acquired in each set of readings or views must be quickly and efficiently digitized so that computer processing can be utilized to provide relatively fast results.
Various problems exist with respect to current analog to digital (A/D) conversion techniques for digitally converting a large number of simulataneously created analog signals, such as those created in a C.T. scan system. For example, one such technique involves dividing the channels, through which the corresponding analog signals are transmitted, into groups, with the channels of each group time sharing an A/D converter. Thus, the analog signals associated with each group are applied to the common A/D converter in a sequential manner so that all of the analog signals transmitted through the channels of a group can be independently converted by the common A/D converter. Frequently, the signal conversion is not identical for each of the signals to the degree necessary to achieve the desired high resolution for a relatively large dynamic range (the latter being of the order of 10.sup.6 to 1). The variabilities among the A/D converters can result in nonuniform readings.
Where such a device is used with a continuous X-ray source, any modulation in the X-ray source over time will create errors since the channels are not all converted simultaneously. The approach described also encounters problems when used with a pulse X-ray source. For example, artifacts due to variable afterglow readings of X-ray pulses are not necessarily treated identically for all of the channels. These interpulse values have an overall effect on the values of the detected analog signals corresponding to the detected X-rays in response to the pulses of X-rays from the source, and the interpulse values should be taken into consideration to provide accurate readings. In addition current leakage of certain storage devices, disposed in each channel, that temporarily store information can create errors in the signal conversion. While some of these problems could be overcome by using a separate A/D converter for each channel, such an approach is impractical because of its prohibitive costs. With the dynamic range of the analog signals provided in each channel of the order of 10.sup.6 to 1 a linear ramp A/D converter is also impractical Many other A/D converter techniques, such as successive approximate A/D conversion, are known, but each is considered to have one or more drawbacks, including inadequate signal resolution and therefore incapable of achieving a digital signal of 20 bits or more.